U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Low permeability formations include diatomites, lipid coals, and oil shales. Formations of low permeability are not amiable to secondary oil recovery methods such as steam, carbon dioxide, or fire flooding. Flooding materials tend to penetrate formations that have low permeabilities preferentially through fractures. The injected materials bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process. Heat injection wells are utilized to provide the heat for such processes.
Heat injection wells can also be useful in decontamination of soils. U.S. Pat. Nos. 5,318,116 and 5,244,310, for example, disclose methods for decontamination of soils wherein heat is injected below the surface of the soil in order to vaporize the contaminates. The heaters of patent '310 utilize electrical resistance of spikes, with electricity passing through the spikes to the earth. Patent '116 discloses heater elements passing through the wellbore to the bottom of the formation to be heated. The wellbore surrounding the heater includes a catalyst bed, which is heated by the heater elements. Heat conductively passes through the catalyst bed to a casing surrounding the catalyst bed, and then radiantly from the casing to the soil surrounding the wellbore. Typical alumina based catalysts have very low thermal conductivities, and a significant temperature gradient will exist through the catalyst bed. This significant temperature gradient will result in decreased heat transfer to the earth being heated at a limited heater element temperature.
U.S. Pat. No. 5,065,818 discloses a heater well with sheathed and mineral insulated ("MI") heater cables cemented directly into the wellbore. The MI cables includes a heating element surrounded by, for example, magnesium oxide insulation and a relatively thin sheathing around the insulation. The outside diameter of the heater cable is typically less than one half of an inch (1.25 cm). The heater well optionally includes a channel for lowering a thermocouple through the cemented wellbore for logging a temperature profile of the heater well. Being cemented directly into the wellbore, a need for a casing (other than the sheathing of the cable) is eliminated, but the outside diameter of the cable is relatively small. The small diameter of the heater cable limits the amount of heat that can be transferred to the formation from the heater cable because the area through which heat must pass at the surface of the cable is limited. A cement will have a relatively low thermal conductivity, and therefore, a greater heat flux at the surface of the cable would result in an unacceptably high heater cable temperature. Multiple heater cables may be cemented into the wellbore to increase the heat transfer to the formation above that which would be possible with only one cable, but it would be desirable to further increase the heat that can be transferred into earth surrounding the heaters.
U.S. Pat. No. 2,732,195 discloses an electrical heater well wherein an "electrically resistant pulverulent" substance, preferably quartz sand or crushed quartz gravel, is placed both inside and outside of a casing of a wellbore heater, and around an electrical heating element inside of the casing. The quartz is placed there to reinforce the casing against external pressures, and a casing that is sealed against the formation is required. The casing adds considerable expense to the installation.
It is therefore an object of the present invention to provide a wellbore heater wherein the heater has a greater surface area at the temperature of the electrical resistance element than those of the prior art, and in which a substantial casing is not required. This heater is useful as a well heater for such purposes as thermal recovery of hydrocarbons and soil remediation.